Process piping is traditionally designed to withstand process pressures and temperatures, as well as any external static and dynamic loads. Advances in metallurgy are permitting the use of thinner wall pipe that allows for more flexible pipework, but at the expense of a higher stress concentration at the branch fittings. A thinner wall pipe with a higher stress concentration at the branch fittings can lead to designs that are more susceptible to a risk of fatigue failure, particularly in systems utilizing pressure-reducing devices with a large pressure drop and large flow rate that act as a loud sound source. The loud sound sources can produce high frequency (acoustically induced) vibration (AIV) of the header pipe, which can potentially lead to fatigue failures at piping discontinuities (e.g. branch fittings) within several minutes or hours.
AIV in piping systems with vapor service is generally caused by acoustic energy created from pressure reducing devices. Examples of such devices include control valves, depressurizing valves, restriction orifices, and pressure safety valves. The typical frequency range of dominant acoustic energy is between 500 and 2000 Hz. The high frequency sound can excite and resonate with the circumferential modes of vibration of header pipe, thus amplifying stress concentrations and create the potential for fatigue failures at piping discontinuities. To mitigate the potential for catastrophic AIV fatigue failure, contoured branch fittings have increasingly been specified for use at branch connections in susceptible piping systems. In the past three decades, contoured branch fittings have eliminated sharp discontinuities at the branch connection, thereby reducing stress concentration and the associated stress intensification factor (SIF).
Contoured branch fittings are, however, primarily designed to resist internal pressure and to reduce stresses under thermal and mechanical loads. Their effectiveness in mitigating AIV is, however, limited. When the anticipated sound power level exceeds the design limit of a contoured branch fitting, the typical solution is to replace the pipe with thicker pipe and associated branch fittings. Other solutions include reduction of sound level at the source, such as splitting the sound energy thru parallel paths or reducing the sound by use of low noise valve trim. These solutions significantly increase the total installation cost, and in many cases can lead to schedule delays.